Contrast adjustment in color TV receivers, particularly those which have inputs for chroma signals red, green and blue (RGB), require amplifiers in which the amplification of respective channels for the respective colors are the same over the entire adjustment range. The frequency band width of wide-band RGB monitors requires circuits which uniformly amplify the wide frequency signals, independently of the respectively set adjustment of the contrast amplification.
Contrast amplifiers of the type to which the present invention relates have, for each channel, a pair of emitter coupled transistors, formed of first and second transistors, and a third transistor, the collector of the third transistor being coupled to the emitter of the first and second transmission of the pair. The emitter of the third transistor is coupled through an emitter-resistor to one terminal of a source of operating voltage.